The present invention relates to a laser system. In particular, the present invention relates to an integrated oscillator-amplifier laser system that is suitable for deep UV generation.
Solid-state laser sources operated at deep UV wavelength around 200 nm are greatly desirable for photo-refractive surgeries. Such a deep UV laser source is expected to be more compact, more reliable, and requires less maintenance compared with excimer laser, which is currently the dominant laser source for photo-refractive surgeries. More importantly, solid-state laser sources can be operated at a much higher repetition rate and with much less energy fluctuation compared with the excimer lasers. Scanning a deep UV laser beam with high repetition rate enables a variety of ablation shapes on a cornea surface and provides a great flexibility for the refractive surgeries. The improved stability in pulse energy from a solid-state UV laser source ensures more accurate and better controllable ablation.
In U.S. Pat. No. 6,031,854 to Lai, a diode pumped cascade laser is proposed for deep UV generation. The second laser employs a short cavity with only a gain medium and a wavelength selection element inside the cavity. When pumped by a laser pulse of 50 ns or shorter, the second laser is gain-switched to produce a pulse of nanosecond duration. This nanosecond laser pulse is then converted to deep UV radiation by a wavelength converter.
In the above approach, a short pump pulse is critical for generating a single short pulse with nanosecond duration and millijole energy. The pulse build-up time is proportional to the laser cavity length and inversely proportional to the net pump pulse energy above the lasing threshold of the cavity. When the pump pulse duration is longer than the build-up time of the laser pulse, a second pulse will appear. This results in smaller energy in the first pulse and thus lowers the conversion efficiency in deep UV generation.
It is well known in the art that a combination of master oscillator-power amplifier system is a common approach to obtain amplified pulses of short duration, in addition to good beam profile and narrow bandwidth. In such a system, the master oscillator is usually a low gain, low power laser to produce a seed pulse of certain specifications. The power amplifier is a high gain, high power laser to amplify the seed pulse to a much higher pulse energy. A number of master oscillator-power amplifier systems are commercially available from, for example, Lambda Physics of Germany and Continuum of Santa Clara, Calif.
The advantage of a master oscillator-power amplifier system is that the oscillator and the amplifier laser cavities can be optimized independently. The system, however, requires two pump sources and two gain media. Also, the system requires additional optics to inject the oscillator seed pulse to the amplifier. As a result, a master oscillator-power amplifier system is usually complicated and expensive.
The present invention contemplates an integrated oscillator-amplifier system for deep UV generation. The system employs a long cavity oscillator to lengthen the pulse build-up time and to control the pulse spectral bandwidth. Meanwhile the system employs a short cavity amplifier to shorten the energy extraction time to produce a single short pulse with good energy extraction efficiency. The system further integrates the oscillator and the amplifier by inserting the amplifier cavity inside the oscillator cavity via a mirror of low reflectivity. As a result, the integrated system has a long build up time to generate a seed pulse in the long cavity oscillator and has a short energy extraction time to generate a short amplified single pulse in the short cavity. Consequently, the integrated system can accommodate a relatively long pump pulse to produce a single short amplified pulse suitable for deep UV laser generation.
According to the present invention, an integrated laser oscillator-amplifier system comprises:
a laser oscillator having a gain medium and a long resonant cavity, said laser oscillator pre-lases to produce a seed pulse when said gain medium is excited with a pump pulse; and
a laser amplifier having said gain medium and a short resonant cavity, said laser amplifier is integrated into said laser oscillator via a mirror with low reflectivity and amplifies said seed pulse to generate a short amplified pulse;
wherein said laser oscillator-amplifier system produces a single amplified laser pulse.
One objective of the present invention is to provide a new and improved laser system for deep UV laser generation.
Another objective of the present invention is to provide a new and improved laser oscillator-amplifier system for generating single pulse of nanosecond duration and millijole energy.
A further objective of the present invention is to provide a new and improved laser oscillator-amplifier system accommodating a pump pulse of approximately 100 ns to generate a single pulse of nanosecond duration.
These and other aspects and advantages of the invention will become more apparent in the following drawings, detailed description and claims.